1. Field of the Invention
The present invention relates to a rechargeable battery charging method that charges with a given amount of set current, to a charging control apparatus, and to a battery pack.
2. Description of the Related Art
Rechargeable battery charging, as represented by lithium-ion battery charging, is performed with a set current and set voltage that are established within the limits of the maximum current and maximum voltage (the protection voltage for preventing over-charging) allowed by the rechargeable battery. Although various different techniques have been proposed to control charging in the past, charging with a set constant current until battery voltage reaches a set voltage and subsequently charging with a set constant voltage (namely, constant current-constant voltage charging) has become the predominantly accepted method.
Incidentally, it turns out that a rechargeable battery has non-negligible internal resistance. During charging, the voltage component generated by the set current flow through the internal resistance is added to the open-circuit voltage and measured as detected voltage. As a result of internal resistance, the measured battery voltage is increased during charging and reduced during discharging. Accordingly, to prevent the detected battery voltage from exceeding the maximum allowable voltage, it is desirable to set the current at the start of charging high or low according to whether the battery voltage is high or low when the battery is not being charged or discharged. It is also known that the internal resistance is high or low depending on whether the battery temperature is low or high respectively. Considering these characteristics, technology disclosed in Japanese Laid-Open Patent Publication 2009-44946 optimizes the set current at the start of charging depending on the value of the battery voltage and the battery temperature. (For example, refer to Table 1 in Japanese Laid-Open Patent Publication 2009-44946.)
Since internal resistance increases with battery degradation, the set current at the start of charging is generally adjusted to a value that is lower than a maximum current flow assumed for a nominal rechargeable battery with a relatively short time-in-service. In the case of a multi-cell battery having a plurality of series-connected battery cells (battery assembly, battery array), the maximum voltage of each cell must be prevented from exceeding the maximum allowable voltage even when cell-to-cell variation has developed due to charge-discharge cycle repetition. Therefore, the set current at the start of charging is adjusted even lower for a multi-cell battery.
In a multi-cell battery, the set voltage and/or set current may be revised according to the maximum cell voltage after charging has been started.
For example, in Japanese Laid-Open Patent Publication 2009-44946, each time the maximum cell voltage exceeds a set voltage, the set voltage and/or set current is reduced and charging continued. This technique suppresses the reduction in charging capacity (effective capacity) that arises in a multi-cell battery due to cell-to-cell voltage variation.
However, when technology disclosed in JP 2009-44946A reduces the set current at the start of charging below the maximum current flow in a nominal rechargeable battery, increase in the time to full-charge becomes unavoidable. Further, this tendency is magnified when the rechargeable battery is a multi-cell battery, and the result is contrary to the quest for reduced charging time.
The present invention was developed reflecting on the situation described above. Thus, it is an object of the present invention to provide a rechargeable battery charging method, charging control apparatus, and battery pack that can complete charging in a short time period by rapidly increasing the set current while keeping battery voltage within a range that does not exceed the maximum allowable voltage.